A signal includes informational content and characteristics other than informational content. For example, an optical signal may have an optical power level, which is a measure of the energy delivered by the optical signal per unit of time. In another example, an electrical signal may have a signal-to-noise ratio, which reflects the relative amounts of signal and noise in a transmission. These signal characteristics that do not represent the informational content of the signal are referred to as parametric information, and may be represented by data, called parametric data.
In some cases, the parametric data is measured on a non-linear scale. For example, optical power is measured in decibels (dB), which is on a logarithmic scale. The equation used to calculate optical power in decibels is dB=10 log (ratio of measured power/reference power).